It is generally required that the piezoelectric substrates of surface acoustic wave devices be great in electromechanical coupling coefficient, low in propagation loss and small in the temperature coefficient of delay time.
With communications made at higher frequencies using digital systems in recent years, there is a growing need for surface acoustic wave devices which are usable in the quasi-microwave band. The center frequency f.sub.0 of surface acoustic wave devices is expressed by the following equation based on the relationship between the acoustic velocity (phase velocity of the surface acoustic wave) V and the line and space range (.lambda./4). EQU f.sub.0 =V/.lambda.
To provide high-frequency-band surface acoustic devices, therefore, research is conducted on methods of diminishing the line and space range by exquisite fabrication of the electrodes, methods of giving higher acoustic velocities by the development of supersonic materials and methods of giving higher acoustic velocities by the application of harmonic waves or higher modes.
However, in diminishing the line and space range, the accuracy of current lithographic techniques for mass production is about 0.6 .mu.m, so that the center frequency is limited to 1.75 GHz, for example, with surface acoustic wave devices wherein lithium tantalate (acoustic velocity 4200 m/s) is used.
On the other hand, a surface acoustic wave device utilizing a higher mode is proposed which has a ZnO film epitaxially grown on the R-plane of sapphire. It is reported that an acoustic velocity of 5300 m/s can be realized by the proposed device with use of the fundamental wave of so-called Sezawa mode which is a higher mode of Rayleigh waves. The value is the limit for the device.
The present applicant has clarified by computer simulation that piezoelectric substrates having an aluminum nitride film formed on a single crystal silicon substrate can be improved in electromethanical coupling coefficient by inclining the C-axis of the aluminum nitride film with respect to a normal to the silicon substrate (U.S. Pat. No. 5,059,847). Nevertheless, the computer simulation merely analyzes Rayleigh waves and clarifies nothing about realization of harmonic waves or higher modes for giving higher acoustic velocities.
Furthermore, a process for forming an aluminum nitride having a tilted C-axis has been proposed in which an electric field of great strength is applied by DC magnetron sputtering (U.S. Pat. No. 4,640,756). The process, however, has the problem of necessitating a large device for producing the great electric field, and yet the process is unable to realize excitation of harmonic waves or higher modes. Additionally, the aluminum nitride film formed by the process is a polycrystalline oriented film and is therefore inevitably inferior to single-crystal films in characteristics.